Chemical Equations & The Law of Conservation of Matter

A chemical equation is a symbolic representation of a chemical reaction.

Equation Example: The burning of methane gas in oxygen is: CH O 2 → CO H 2 O

Review: Element Symbols All elements are represented by a 1 or 2 letter symbol  For example C = Carbon Ne = Neon O = Oxygen The symbols are shown on the periodic table

Review: Chemical Formulas Shows the elements & number of atoms of each element in a molecule H 2 SO 4  Elements Hydrogen: 2 atoms Sulfur: 1 atom Oxygen: 4 atoms  7 atoms total Subscript

Coefficients A formula may begin with a number. If there is no number, then “1” is understood to be in front of the formula. –This number is called the coefficient. –The coefficient represents the number of molecules of that compound or atom needed in the reaction. –For example: 2H 2 SO 4 The coefficient 2 indicates that there are 2 molecules of Sulfuric Acid (H 2 SO 4 )

Coefficients 2H 2 SO molecules of Sulfuric Acid –A coefficient is distributed to ALL elements in a compound 2 x H 2 (for a total of 4 H atoms) 2 x S (for a total of 2 S atoms) 2 x O 4 (for a total of 8 O atoms)

Practice: Look at each compound. How many atoms of each element are in the formula? » 2NH 3 » 3H 2 O » CH 4 » 2C 6 H 7 O 2 (OH) 3

Now you are ready to read a chemical equation!

Reading Chemical Equations Each side of an equation represents a combination of chemicals experiencing a chemical reaction. The combination is written as a set of chemical formulas, separated by + symbols. CH O 2 → CO H 2 O

Reading Chemical Equations The two sides of the equation are separated by an arrow, which stands for the word “yield” (which means makes). –The combination of chemicals before the reaction are on the left side of the arrow –The right side indicates the combination of chemicals after the reaction. –These parts have proper names.

yield

Example of a chemical reaction represented by an equation: In this reaction, four sodium atoms and a molecule of oxygen (O 2 ) react to yield two molecules of Na 2 O 4Na + O 2 → 2Na 2 O

Balancing Equations The Law of Conservation of Matter states that…... Matter is neither created or destroyed during a chemical reaction. This means that each side of the equation must represent the same quantity of each element; in other words, each side must have the same number of each kind of atom.

Reactants  Products Amount of matter = Amount of matter Mass of Reactants = Mass of Products

Balancing Equations Na + O 2 → Na 2 O In order for this equation to be balanced, there must be equal amount of Na on the left hand side and on the right hand side. Right now, there is 1 Na atom on the left but 2 Na atoms on the right. We solve this problem by putting a coefficient of 2 in front of the Na on the left hand side, Like this: 2Na + O 2 → Na 2 O

Balancing Equations 2Na + O 2 → Na 2 O There are 2 Na's on the left and 2 Na's on the right. But what about the O? We now must check to see if the O's are balanced on both sides of the equation. On the left hand side there are 2 O's and the right hand side only has one. This is still an unbalanced equation. To fix this we must put a 2 in front of the Na 2 O on the right hand side. Now our equation reads: 2Na + O 2 → 2Na 2 O

Balancing Equations 2Na + O 2 → 2Na 2 O Notice that the 2 on the right hand side is "distributed" to both the Na 2 and the O. Currently the left hand side of the equation has 2 Na's and 2O's. The right hand side has 4 Na's total and 2 O's. Again, this is a problem, there must be an equal amount of each chemical on both sides. To fix this let's add 2 more Na's on the left side. The equation will now look like this, and is now balanced: 4Na + O 2 → 2Na 2 O

Here is another example of a balanced chemical equation: 4 hydrogen atoms + 2 oxygen atoms yields 2 molecules of water (each with 2 H atoms and one O atom)

Lets try this one together: Is it balanced?

Follow along with your notes to balance this equation: Ca + O 2 CaO

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